Multi-element antenna systems, including multiple-input multiple-output (MIMO) systems, may increase the capacity and diversity in wireless communication systems. Several communication techniques, including space-time coding and spatial multiplexing, have been developed to exploit the potential of MIMO systems. Spatial multiplexing focuses on the rate advantage whereas space-time coding focuses on the diversity advantage of MIMO systems.
Most MIMO systems use a statistical channel model that is an idealized abstraction of spatial propagation characteristics and assumes independent and identically distributed (i.i.d.) fading between different transmit-receive antenna pairs. In practice, however, the channel coefficients between different transmit-receive antenna pairs may exhibit correlation due to closely spaced antennas and clustered scattering typical in realistic environments. In such conditions, the capacity of MIMO channels can be substantially lower, depending on the level of correlation.